Various types of force sensors have been used to control motions of robots and industrial machines. Also, a downsized force sensor has been incorporated as a man-machine interface of an input device for electronics. In order to reduce dimensions and cost, a force sensor to be used in the above-described applications is required to be simple in structure as much as possible and also to independently detect force for each coordinate axis in a three dimensional space.
At present, generally-used multi-axis force sensors are categorized into such a type that a specific directional component of force exerted on a mechanical structure is detected as displacement occurring at a specific site and such a type that the force is detected as a mechanical strain occurring at a specific site. A force sensor using a capacitive element is a representative sensor of the former displacement detection types. This force sensor has a capacitive element constituted by a pair of electrodes and detects displacement occurring at one of the electrodes by the force exerted on the basis of a capacitance value of the capacitive element. A multi-axis force sensor using a capacitive element has been disclosed, for example, in Japanese Unexamined Patent Publication No. 2004-325367 (U.S. Pat. No. 7,219,561) and Japanese Unexamined Patent Publication No. 2004-354049 (U.S. Pat. No. 6,915,709).
On the other hand, a strain gauge-type force sensor is a representative sensor of the latter type which detects a mechanical strain. This strain gauge-type force sensor detects a mechanical strain resulting from the force exerted as a change in electric resistance of strain gauges and others. The strain gauge-type multi-axis force sensor has been disclosed, for example, in Japanese Unexamined Patent Publication No. 8-122178 (U.S. Pat. No. 5,490,427).
However, in any of the multi-axis force sensors disclosed in the above-described Patent Documents, a mechanical structure is inevitably made thick. Therefore, it is difficult to make thin the sensor in its entirety.
On the other hand, in the fields of robots, industrial machines, input devices for electronics and others, it is desired to develop a thinner-type force sensor. Thus, an object of the present invention is to provide a force sensor which is simple in structure and can be made thinner.